说明：  使用Qt编写的计算器程序，可以支持浮点数运算； 信号和槽的简单运用； 思路：先将字符串转化成浮点数和符号；再将中缀表达式转化为逆波兰式；最后直接计算。
(The calculator program written by QT can support floating-point operation; Simple use of signals and slots; Idea: first convert string to floating-point numbers and symbols; then convert infix expression to inverse Polish; finally, directly calculate.)